The present invention relates to a conductive member used for a charging unit, a development unit, a transfer unit, and a cleaning unit, which are used for electrophotographic apparatus or an electrostatic recording process, and an electrophotographic apparatus including a charging unit, a development unit, a transfer unit, and a cleaning unit, each of which uses the conductive member.
An electrophotographic apparatus or an electrostatic recording apparatus, such as a copying machine, a facsimile, or a printer is operated in accordance with a printing method, which includes a charging step of uniformly charging the surface of a photosensitive body, an electrostatic latent image forming step of projecting an image from an optical system to the photosensitive body, to erase charges in a portion irradiated with light, thereby forming an electrostatic latent image, a development step of sticking toner on the latent image, to form a toner image, and a transfer step of transferring the toner image to a recording medium such as a paper sheet.
In this printing process, the initial step of charging a photosensitive body has been generally performed by using a corona discharging method. The corona discharging method, however, is undesirable from the viewpoints of safety and maintenance of the apparatus because the corona discharging method needs the supply of a high voltage such as 6 to 10 kV. The corona discharging method also presents an environmental problem because a hazardous substance such as ozone or NOx occurs during discharge of corona.
To solve the above problems, various attempts have been made to develop a new charging method capable of performing charging at a voltage lower than that required for the corona discharging method and suppressing occurrence of a hazardous substance such as ozone. For example, a contact type charging method shown in FIG. 3 has been proposed as an alternative to the corona charging method. Referring to FIG. 3, a roller-shaped charging member (charging roller) 14, to which a voltage has been applied from a power source 16, is brought into contact with a body 15 to be charged such as a photosensitive body at a specific pressure, to thereby charge the body 15 to be charged.
With respect to the development step, as a development method of supplying a non-magnetic one-component developer to a photosensitive drum or the like on which a latent image has been formed, to stick the developer to the latent image on the photosensitive drum, thereby visualizing the latent image, a pressure development method has been known, for example, from U.S. Pat. Nos. 3,152,012 and 3,731,146. This pressure development method can eliminate the need of use of any magnetic material, and is thereby advantageous in simplifying the structure of the apparatus and easily reducing the size of the apparatus, and further easily realizing development using a plurality of kinds of colored toner.
Referring to FIG. 2, the pressure development method is performed by bringing a development roller 6, on which toner (non-magnetic one-component developer) has been supported, into contact with a latent image support 5 such as a photosensitive drum on which an electrostatic latent image has been formed, to stick the toner to the latent image on the latent image support 5, thereby developing the latent image. Accordingly, the development roller 6 must be rotated while certainly holding the state that the development roller 6 is brought into close-contact with the latent image support 5 such as a photosensitive drum, and therefore, the development roller 6 is required to be made from a conductive elastic body.
A transfer unit, used in the transfer step, for transferring a toner image, which has been developed with toner for visualization, from a latent image support to a transfer medium such as a paper sheet, has been generally configured to transfer a toner image to a transfer medium by charging the transfer medium with the use of a corona charger. The corona discharge, however, has the above-described problems associated with occurrence of ozone and the need of a high voltage power source. To solve such problems, there has been known a transfer unit shown in FIG. 2, wherein a transfer medium 8 is charged by using a bias roller (transfer roller) 9 made from a conductive rubber. In this method, to enhance the transfer efficiency and obtain a uniform transfer image, it is required to set a specific nip width between the transfer roller 9 and a photosensitive drum 5 and reduce a pressure applied between the roller and the drum, and to meet such a requirement, a very soft conductive rubber must be used as the material forming the transfer roller 9. It is to be noted that, as shown in FIG. 2, the toner image transferred to the transfer medium 8 is heated by a fixing unit 13 to be thus fixed to the transfer medium 8.
After transfer of a toner image, toner remaining on a photosensitive drum is removed by a cleaning unit. Such a cleaning unit has been generally configured to scrape toner by an edge of a blade made from a urethane rubber or the like pressed on a photosensitive body. The use of the blade, however, has a problem that since a large frictional force occurs against a photosensitive body, a drive force becomes large, the photosensitive drum is liable to be damaged by the edge of the blade, and in the worst case, the cleaning operation becomes impossible because of damage of the rubber blade. To solve such a problem, a cleaning unit 12 using a cleaning roller 11 to which a voltage is applicable (see FIG. 2) has been proposed, wherein residual toner is directly removed from the surface of the photosensitive drum 5 by the cleaning roller 11. Alternatively, a cleaner-less method has been proposed, in which toner is forcibly charged and then recovered by a development roller or the like. In the case of adopting the cleaning method using the cleaning unit 12, the cleaning roller 11 constituting the cleaning unit 12 also requires the same characteristics as those required for the transfer roller.
The above-described charging roller, development roller, transfer roller, or cleaning roller has been configured as a conductive member obtained by forming a resin layer on the surface of an elastic layer such as a rubber material or a urethane form. The formation of the resin layer is for ensuring a surface smoothness, adjusting a surface resistance, and improving a charging characteristic. The resin layer is typically formed by coating the elastic layer with a solution of a resin selected from an acrylic resin, a urethane resin, nylon, a polyethylene resin, an epoxy resin, a polyester resin, a polyether resin, a polystyrene resin, a phenol resin, an ABS resin, a polyamide resin, and a urethane modified acrylic resin by a dipping method or a spraying method.
The above-described conventional conductive member, however, has the following problems:
Initial Fog/Fog after Repeated Printing
In the recent electrophotographic field, there have been strong demands to enhance the image quality, lower the cost (lower the voltage or reduce the size of a member), increase the operational speed, and improve the durability, and to meet such demands, attempts have been made to improve electrophotographic apparatuses. These apparatuses thus improved to meet the above-described demands, however, may often cause a problem associated with initial image defects (such as initial fog) and image defects after repeated printing (such as fog after repeated printing) due to the effect of a conductive member used for a charging roller, a development roller, a transfer roller, or a cleaning roller. The reason why the image defects such as fog occur due to the effect of the conductive member, however, has not been basically revealed, and at the present time, any countermeasure capable of sufficiently solving such a problem has not been proposed yet.
Adhesiveness to Photosensitive Body (OPC)
A conductive member used for a charging roller, a development roller, a transfer roller, or a cleaning roller is put for a long time in the state being in contact with a photosensitive body represented by an organic photoconductor (OPC), so that adhesion (stickiness) may occur between the photosensitive body and the surface of the conductive member, resulting in peeling of a coating film from the surface of the conductive member or contamination of the photosensitive body. In particular, to meet the recent requirement for electrophotographic apparatuses to be usable in various environments, it has been strongly required to develop a conductive member (particularly, a resin layer on the surface thereof) excellent in anti-adhesiveness to a photosensitive body, that is, relesability from the photosensitive body even under a severe condition in terms of temperature and humidity.
Trace of Contact Portion (Nip Mark)
Since a conductive member is put for a long time in the state being in contact with a photosensitive body as described above, a trace of the contact portion (nip mark) therebetween may remain on the conductive member under a severe condition in terms of temperature and humidity, to cause a problem that the trace may appear in an image periodically in the rotation direction of the conductive member. In particular, the problem becomes significant for a charging roller used for the contact type charging method.
In view of the foregoing, the present invention has been made, and an object of the present invention is to provide a conductive member capable of solving the above-described problems associated with the [initial fog/fog after repeated printing], [adhesiveness to OPC], and [nip mark], thereby certainly performing, even under a severe condition, desirable charging, development, transfer, and cleaning operations to stably obtain desirable images, and to provide an electrophotographic apparatus using the conductive member.
The present inventors have studied to develop a conductive member used for a charging roller, a development roller, a transfer roller, or a cleaning roller, which member is capable of achieving the above object, and found that the above object can be achieved, as will be described below, by providing a conductive member including an elastic layer and at least one resin layer formed on the elastic layer, wherein the surface charge potential characteristic of the outermost resin layer is specified as follows: namely, when the surface of the outermost resin layer being in a state not containing any conductive agent is charged due to corona discharge generated by applying a voltage of 8 kV to a corona discharger disposed with a gap of 1 mm put therebetween, a surface potential after an elapse of 0.3 sec is in a range of 50 V or less and a surface potential after an elapse of 10 sec is in a range of 5 V or less.
Initial Fog/Fog after Repeated Printing
As a result of examination made by the present inventors, it has been found that the above-described [initial fog/fog after repeated printing] is mainly dependent on the resin layer of the conductive member, particularly, on the outermost resin layer. In particular, it has been found that the occurrence of the [initial fog] can be significantly prevented by specifying the surface charge potential characteristic of the outermost resin layer such that the surface potential after an elapse of 0.3 sec is in a range of 50 V or less and the surface potential after an elapse of 10 sec is in a range of 5 V or less. It is to be noted that the measurement of the surface charge potential characteristic is, as described above, performed by charging the surface of the outermost resin layer (which is in a state not containing any conductive agent) due to corona discharge generated by applying a voltage of 8 kV to a corona discharger disposed with a gap of 1 mm put therebetween. With respect to the [fog after repeated printing], it has been found that in the case where the occurrence of initial fog is prevented, although occurrence of fog after repeated printing cannot be perfectly prevented, the degree of fog after repeated printing becomes significantly smaller than that in the case where the initial fog occurs.
Meanwhile, it has been also found that, to prevent occurrence of the [fog after repeated printing], not only the above surface charge potential characteristic but also the toner adhesion property and friction property of the outermost resin layer of the conductive member become important. To be more specific, if the degree of adhesion of toner on the conductive member such as a charging roller, a development roller, a transfer roller, or a cleaning roller becomes large or if the toner adhering on the conductive member is less removed, a film-like toner layer is formed on the conductive member or the toner is fused thereon, to damage the surface of the conductive member, thereby leading to fog. On the other hand, if the friction property of the outermost resin layer is high, a shear stress between a photosensitive body and the outermost resin layer becomes excessively large, to damage the surface of the outermost resin layer by sagging, wrinkle, or piercing, thereby leading to occurrence of fog. From this viewpoint, the present inventors have further examined and found that the occurrence of the [fog after repeated printing] can be significantly effectively suppressed by specifying the friction property of the conductive member such that a friction coefficient of the conductive member is in a range of 1 or less, wherein the friction coefficient is measured by bringing the conductive member in press-contact with a cloth of 70 mesh containing 100% of cellulose (density: 30 g/m2) at a load of 100 gf and sliding the conductive member against the cloth.
By the way, the reason why the surface potential characteristic using corona discharge is taken as an index for evaluating the degree of fog is as follows. The phenomenon xe2x80x9cfogxe2x80x9d, which is a so-called image defect, occurs due to a mechanical, environmental, or electrical failure, or a failure of a charging roller, a development roller, a transfer roller, or a cleaning roller. Of these causes, the major one common to all of the rollers is an electrical failure due to surface remaining charges. To be more specific, each of these rollers plays a role when a voltage is applied thereto, and if charges remain on the roller in a state that no voltage is applied to the roller, electrical ununiformity may occur on the roller when a voltage is next applied to the roller. The electrical ununiformity on the conductive roller may cause critical image defect, that is, fog such as xe2x80x9ca white spotxe2x80x9d, xe2x80x9ca black spotxe2x80x9d, xe2x80x9ca lateral streakxe2x80x9d, or xe2x80x9clongitudinal streakxe2x80x9d. Accordingly, it is required to make charges remaining on the surface of the roller as small as possible. For this reason, the ability of preventing occurrence of the xe2x80x9cfogxe2x80x9d can be evaluated by using the surface charge attenuation characteristic accompanied by surface potential measurement using corona discharge, and according to the present invention, such a surface charge attenuation characteristic is evaluated and adjusted.
In general, a voltage is applied to a charging roller by supplying the voltage to a shaft of the charging roller, and in this case, it is required that at the moment of supplying the voltage, charges migrate to the surface of the charging roller, to charge a photosensitive body. If such an ability is insufficient, the photosensitive body cannot be sufficiently charged, to cause an image defect, particularly, fog. This charging ability has a relation with an electrostatic capacity and resistance of the roller, but it cannot be sufficiently correlated therewith. As a result of examination of such a charging ability, made by the present inventors, it has been found that the charging ability can be substantially linearly evaluated by measuring a responsiveness to charges on the surface of a charging roller, that is, a surface potential due to corona discharge.
The reason why the charge potential characteristic is evaluated in the state that the outermost resin layer does not contain any conductive agent is as follows. In general, a conductive roller such as a charging roller, a development roller, a transfer roller, or a cleaning roller has a resistance distribution that the resistance becomes higher in the direction from the shaft to the outer layer, and such a resistance distribution has a relation with an anti-leakage performance, a charging performance of toner, and the like. Accordingly, the charge permeability of surface charges of the roller is substantially dependent on only the surface layer. By the way, the surface layer of the roller often contains a conductive agent. In this case, the conductive agent may contribute to the removal of the surface charges to some extent; however, if the conductive agent is a filler in the form of particles, the particles of the filler are little in contact with each other from the microscopic viewpoint, with a result that the stay of the charges in a resin of the surface layer cannot be perfectly eliminated. Eventually, unless the resin of the surface layer has an excellent charge permeability, a perfect state with no residual charges cannot be obtained by the presence of the filler. On the other hand, if the conductive agent is an ionic conductive agent, since a concentration gradient of the ionic conductive agent occurs in the resin of the surface layer, the surface layer has portions in which the ionic conductive agent is little present. Eventually, like the case of using a filler as a conductive agent, the charge permeability of the resin of the surface layer becomes important. Accordingly, the charge permeability of the resin of the outermost resin layer becomes the most important parameter, and therefore, it becomes reasonable to evaluate the surface potential characteristic in the state that the outermost resin layer does not contain any conductive agent.
With respect to the charging operation, as a result of examination made by the present inventors, it has come to be revealed that the image characteristic, particularly, fog is greatly dependent on the charging characteristic of a charging roller, particularly, on the charging characteristic of the outermost resin layer of the charging roller. In this case, it is reasonable to evaluate the charging characteristic of a resin of the outermost resin layer formed on the surface of the charging roller. In other words, it is undesirable to evaluate the charging characteristic of the outermost resin layer containing other components such as a conductive agent. In this way, the charge potential characteristic must be evaluated in the state that the outermost resin layer does not contain any conductive agent.
The reason why the charging potential after an elapse of 10 sec since charging by corona discharge is evaluated is as follows. In this evaluation test, a large voltage such as 8 kV generated by corona discharge is supplied to the surface of a conductive roller by corona discharge (note: the voltage applied to the conductive roller is regarded to be smaller than 8 kV); however, in general, such a large voltage is not supplied to the conductive roller of an actual apparatus. In this evaluation test, the above-described large voltage is supplied to the conductive roller from the viewpoints of measurement accuracy, repeatability, and measurement method. Taking into account the above circumstances, according to the present invention, the surface potential after an elapse of 10 sec, which is required to release the charges given by applying such a large voltage, is evaluated. Additionally, experience has showed that this evaluation is reasonable.
Adhesiveness to OPC
The problem associated with the [adhesiveness to OPC] can be solved, as described above, by enhancing the releasbility of a conductive member, and as a result of examination made by the present inventors, it has been found that it is possible to effectively solve the problem associated with the [adhesiveness to OPC] and hence to effectively prevent occurrence of inconveniences such as peeling of a film and contamination of a photosensitive body by setting the contact angle between the surface of a conductive member, that is, the surface of the outermost resin layer and water to a value in a range of 90xc2x0 or more.
Nip Mark
The problem associated with the [nip mark] is greatly dependent on a dynamic characteristic of the outermost resin layer forming the surface of a conductive member, and as a result of examination of this point, it has been found that the problem associated with the [nip mark] can be solved by optimizing the physical characteristic of the outermost resin layer forming the surface of the conductive member. Specifically, it has been found that the problem associated with the [nip mark] can be solved by setting the physical characteristic of the outermost resin layer such that after the outermost resin layer is stretched to a length being 1.5 times the original length under an environment with a temperature of 40xc2x0 C. and a humidity of 95% RH and is left for one day in such a state, a residual elongation of the outermost resin layer is in a range of 50% or less.
Accordingly, the present invention provides a conductive member used for an electrophotographic apparatus, including an elastic layer and at least one resin layer formed on the elastic layer, wherein when the surface of the outermost resin layer of the at least one resin layer, which outermost resin layer is in a state not containing any conductive agent, is charged due to corona discharge generated by applying a voltage of 8 kV to a corona discharger disposed with a gap of 1 mm put between the surface of the outermost resin layer and the corona discharger, a surface potential of the outermost resin layer after an elapse of 0.3 sec is in a range of 50 V or less and a surface potential of the outermost resin layer after an elapse of 10 sec is in a range of 5 V or less.
The present invention also provides, as preferable embodiments of the above-described conductive member, the following members (1) to (3):
(1) a conductive member, wherein a friction coefficient of the conductive member, which is measured by bringing the conductive member in press-contact with a cloth of 70 mesh containing 100% of cellulose (density: 30 g/m2) at a load of 100 gf and sliding the conductive member against the cloth, is in a range of 1 or less, particularly, 0.5 or less;
(2) a conductive member, wherein a contact angle between the surface of the outermost resin layer and water is in a range of 90xc2x0 or more; and
(3) a conductive member, wherein a residual elongation of the resin material forming the outermost resin layer is specified such that when a film made from the resin material and having the same thickness as that of the outermost resin layer is stretched to a length being 1.5 times the original length under an environment with a temperature of 40xc2x0 C. and a humidity of 95% RH and is left for one day in such a state, a residual elongation of the film is in a range of 50% or less.
The present invention also provides, as electrophotographic apparatuses using the above-described conductive member, the following electrophotographic apparatuses (1) to (4):
(1) an electrophotographic apparatus including a charging unit including a charging member to be brought into contact with a body to be charged for charging the body to be charged, and means for applying a voltage between the body to be charged and the charging member, wherein the charging member of the charging unit is configured as the conductive member of the present invention:
(2) an electrophotographic apparatus including a development unit operated to support a developer on the surface of the conductive member of the present invention so as to form a thin film of the developer, and bring the conductive member into contact with a latent image support on the surface of which an electrostatic latent image has been formed so as to stick the developer on the electrostatic latent image formed on the surface of the latent image support, thereby visualizing the electrostatic latent image;
(3) an electrophotographic apparatus including a transfer unit operated to charge a transfer medium by using the conductive member of the present invention, visualize an electrostatic latent image by a developer, and transfer the developer from the visualized electrostatic latent image to the transfer medium; and
(4) an electrophotographic apparatus including a cleaning unit operated to remove a developer remaining on a latent image support by using the conductive member of the present invention.